Only one aromatic polyimide dense film which has been hitherto known to be usable as a gas-separating film, is a dense film prepared from pyromellitic acid and an aromatic diamine. However, this known aromatic polyimide dense film is unsatisfactory in its gas-separating property, because the dense film exhibits a maximum ratio of gas permeability coefficient of hydrogen gas (PH.sub.2) to that of carbon monoxide gas (PCO) of about 75 and a maximum ratio of gas permeability coefficient of oxygen gas (PO.sub.2) to that of nitrogen gas (PN.sub.2) of about 4.5.
Generally, the aromatic polyimide has an excellent heat-resisting property, a superior chemical-resisting property and satisfactory mechanical properties. Accordingly, it is strongly desired by industry to provide an aromatic polyimide film having a satisfactory gas-separating property and excellent durability at an elevated temperature.